Biophysical properties of voltage-gated Na+ channels in frog parathyroid cells and their modulation by cannabinoids

Abstract

The membrane properties of isolated frog parathyroid cells were studied using perforated and conventional whole-cell patch-clamp techniques. Frog parathyroid cells displayed transient inward currents in response to depolarizing pulses from a holding potential of –84·mV. We analyzed the biophysical properties of the inward currents. The inward currents disappeared by the replacement of external Na+ with NMDG+ and were reversibly inhibited by 3· mol·l –1 TTX, indicating that the currents occur through the TTX-sensitive voltage-gated Na+ channels. Current density elicited by a voltage step from –84·mV to –24·mV was –80·pA·pF–1 in perforated mode and –55·pA·pF–1 in conventional mode. Current density was decreased to –12·pA·pF–1 by internal GTP S (0.5·mmol·l –1), but not affected by internal GDP S (1·mmol·l -1). The voltage of half-maximum (V1/2) activation was –46·mV in both perforated and conventional modes. V1/2 of inactivation was –80·mV in perforated mode and –86·mV in conventional mode. Internal GTP S (0.5·mmol·l –1) shifted the V1/2 for activation to –36·mV and for inactivation to –98·mV. A putative endocannabinoid, 2-arachidonoylglycerol ether (2-AG ether, 50· mol·l –1) and a cannabinomimetic aminoalkylindole, WIN 55,212-2 (10· mol·l –1) also greatly reduced the Na+ current and shifted the V1/2 for activation and inactivation. The results suggest that the Na+ currents in frog parathyroid cells can be modulated by cannabinoids via a G protein-dependent mechanism.